Internal stresses in metallic workpieces can be reduced by subjecting the workpiece to vibrations causing a decrease of internal stresses.
These workpieces are normally excited with a number of revolutions in the range of from 1200 to 6000 rpm or also up to 12,000 rpm for stress relief. These numbers of revolutions correspond to excitation frequencies of 20 to 100 Hz and 200 Hz, respectively. It is, however, difficult to ascertain the excitation frequencies with which an optimum and purposeful reduction of the internal stresses can be achieved. In the publications GB 20 88 269, U.S. Pat. No. 3,677,831, U.S. Pat. No. 4,446,733 and EP 0 261 273 B1 methods are described by means of which suitable frequencies can be determined for exciting the workpieces. In the case of these methods, a test run within the operating range, which will also be referred to as macroscanning hereinbelow, is first carried out for determining at which numbers of revolutions or excitation frequencies the workpiece starts to vibrate strongly (resonant frequency). The vibration behavior is normally determined with the aid of an accelerometer which is secured to the workpiece. For eliminating the internal stresses, the workpiece is then subjected to vibrations at frequencies at which the workpiece shows resonances during the test phase. If the workpieces in question have a complicated three-dimensional structure, an acceleration value/excitation frequency diagram will normally contain so many peak values that a selection has to be made for the stress relief phase.
The method according to U.S. Pat. No. 4,446,733 only uses the harmonic vibration in the ultrasonic region for stress relief. In the case of the method according to EP 0 261 273 B1, the harmonic vibrations lying outside of the operating range are only used as a basis for calculating the excitation frequencies from the low operating frequency range. The stress relief is, consequently, only carried out with low frequencies.
It is known that the motions in the microscopic region which are necessary for stress relief are excited not directly by the excitation frequencies of the vibrator, but only by the harmonics of the excitation frequencies. When excitation frequencies have been determined previously, it has been assumed that excitation frequencies, which resulted in particularly high peak values in the test run, will also result in a particularly strong excitation of the workpiece in the microscopic region, which is the region of interest. Practical experience has, however, shown that not all the revolution values which achieved amplitude maxima in the test run will also result in a high excitation in the frequency range of interest.